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Microsoft preps fix for IE 8 flaw that makes safe sites unsafe

Third time's the charm

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Microsoft will release an update intended to rid Internet Explorer 8 of a vulnerability that can enable serious security attacks against websites that are otherwise safe.

The change, which will be introduced in June, will be the third time in six months that Microsoft has tweaked a feature used to filter out XSS, or cross-site scripting filter, attacks against websites. The filter, which Microsoft introduced with the release of IE 8, is designed to strip out malicious commands that exploit the vulnerabilities, which plague many websites.

As The Register reported in November, the new XSS filter could be exploited to introduce XSS attacks on sites that otherwise weren't vulnerable. Microsoft has twice made changes to the feature, once in January and again in March, but last week, researchers at the Black Hat Security Conference in Barcelona showed the filter still injected threats into sites that included Google, Wikipedia, Twitter and even Microsoft's own Bing.

"This issue manifests when malicious script can 'break out' from within a construct that is already within an existing script block," David Ross, of Microsoft Security Response Center, said here. "While the issue identified and addressed in MS10-002 was identified to exist on high-profile websites, thus far real-world examples of the SCRIPT tag neutering attack scenario have been hard to come by."

XSS exploits allow attackers to inject malicious code or content into trusted websites by convincing victims to click on booby-trapped links. Because the links contain the well-known domain names, they rarely arouse suspicion, as was the case recently when admins from the Apache Foundation visited tainted links that exposed their login credentials and led to a serious security lapse.

Features like Microsoft's XSS filter, or a similar protection offered in the NoScript add-on for Mozilla Firefox, are designed to prevent such attacks. ®

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